linux-kernel - [PATCH v3 1/4] drm: Expand max DRM device number to full MINORBITS

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Date:   Tue, 6 Sep 2022 16:01:14 +0200
From:   Michał Winiarski <michal.winiarski@...el.com>
To:     <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
CC:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Simon Ser <contact@...rsion.fr>,
        Matthew Wilcox <willy@...radead.org>,
        Michał Winiarski <michal.winiarski@...el.com>
Subject: [PATCH v3 1/4] drm: Expand max DRM device number to full MINORBITS

Having a limit of 64 DRM devices is not good enough for modern world
where we have multi-GPU servers, SR-IOV virtual functions and virtual
devices used for testing.
Let's utilize full minor range for DRM devices.
To avoid regressing the existing userspace, we're still maintaining the
numbering scheme where 0-63 is used for primary, 64-127 is reserved
(formerly for control) and 128-191 is used for render.
For minors >= 192, we're allocating minors dynamically on a first-come,
first-served basis.

Signed-off-by: Michał Winiarski <michal.winiarski@...el.com>
---
 drivers/gpu/drm/drm_drv.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 8214a0b1ab7f..9432b1619602 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -122,6 +122,12 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
 	minor->type = type;
 	minor->dev = dev;
 
+	/*
+	 * DRM used to support 64 devices, for backwards compatibility we need to maintain the
+	 * minor allocation scheme where minors 0-63 are primary nodes, 64-127 are control nodes,
+	 * and 128-191 are render nodes.
+	 * After reaching the limit, we're allocating minors dynamically - first-come, first-serve.
+	 */
 	idr_preload(GFP_KERNEL);
 	spin_lock_irqsave(&drm_minor_lock, flags);
 	r = idr_alloc(&drm_minors_idr,
@@ -129,6 +135,8 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
 		      64 * type,
 		      64 * (type + 1),
 		      GFP_NOWAIT);
+	if (r == -ENOSPC)
+		r = idr_alloc(&drm_minors_idr, NULL, 192, 1 << MINORBITS, GFP_NOWAIT);
 	spin_unlock_irqrestore(&drm_minor_lock, flags);
 	idr_preload_end();
 
-- 
2.37.3